The present invention relates to security and authentication and, in particular, it concerns polymer layers having independent latent images visible in polarized light and methods for generating such layers. It is known to produce various copy-resistant features in manufactured products to guard against forgery. Typical examples of such features are water-marks, holograms, micro-printing and embedded metallic strips. An alternative type of protection could be provided by features which, although not immediately visible, may readily be checked to confirm the authenticity of a product. One attempt at such a feature employs implanted liquid crystal optical elements, which are configured to affect the polarization of transmitted light such that, when viewed through a polarizer, an identifiable pattern is seen. Examples of such a structure are described in European Patent Publications EP 689 065 A1 and EP 689 084 A1. The structures disclosed are multi-layer structures forming optical components based on a photo-crosslinked liquid crystal xe2x80x9cmonomerxe2x80x9d. Formation of the optical components in the liquid crystal polymers requires multiple coating of the substrate material with subsequent photochemical, electrical, magnetic and mechanical processing. The structures described in the aforementioned documents suffer from considerable practical limitations. Firstly, formation of multi-layer liquid crystal components with separate processing of each layer requires complex and expensive manufacturing equipment and materials. Secondly, the various layers making up the liquid crystal components differ in their optical, chemical and mechanical properties. These differences may lead to de-lamination and/or image quality deterioration when the structure is exposed to thermal, chemical or mechanical stresses. Furthermore, the complexity of the multi-layer structure rules out its use in certain applications such as, for example, flexible film. It is also important to note that liquid crystal images are normally visible when viewed directly by the human eye. The formation of a single latent image within a polymer material is described in U.S. Pat. No. 6,124,970 to Karassev, et al., which teaches producing independent latent images by superimposing two independent polymer layers to form a multi-layer structure. A shortcoming of the aforementioned method is that the complexity of the multi-layer structure rules out its use in certain applications, as mentioned above.
There is therefore a need for latent image products in which independent latent images, visible in polarized light, are provided within a single polymer layer, which are convenient and economical to produce, and which are structurally simple and mechanically stable such that they can be used in a wide range of practical applications. It would also be advantageous to provide straightforward and economical methods for producing such products.
The present invention is an authenticatable product including a layer of polymer material which has independent latent images invisible when viewed in unpolarized light which become separately visible with the aid of polarizers. The invention also relates to methods for producing such polymer layers.
According to the teachings of the present invention there is provided, an authenticatable product comprising a non-opaque latent image layer of anisotropic polymer material having background optical properties with a given preferred direction of anisotropy and a given proportion of crystallinity, the latent image layer being treated so as to include: (a) a first localized modification in the latent image layer, the first localized modification being associated with a first latent image and modified in at least one of the given preferred direction of anisotropy and the given proportion of crystallinity; and (b) a second localized modification in the latent image layer, the second localized modification being associated with a second latent image and modified in at least one of the given preferred direction of anisotropy and the given proportion of crystallinity such that, when viewed directly, the first latent image and the second latent image are indistinguishable from the remainder of the latent image layer and, under polarized visualization, a maximum contrast between the first latent image and the remainder of the latent image layer is actualized at a different orientation of polarized visualization than a maximum contrast between the second latent image and the remainder of the latent image layer.
According to the teachings of the present invention at least part of the first latent image and at least part of the second latent image coexist within the same area of the latent image layer.
According to a further feature of the present invention the first localized modification and the second localized modification have mechanical properties substantially identical to mechanical properties of the remainder of the latent image layer.
According to a further feature of the present invention the latent image layer is designed such that optical properties of the latent image layer are substantially unchanged by exposure to temperatures of up to 150 degrees Centigrade.
According to a further feature of the present invention the latent image layer is substantially transparent.
According to a further feature of the present invention the latent image layer is highly transparent.
According to a further feature of the present invention the latent image layer is implemented as part of a laser die cut label.
According to a further feature of the present invention, there is also provided, an authenticatable product comprising a non-opaque latent image layer of polymer material treated so as to include a first latent image and a second latent image such that, when viewed directly, the first latent image and the second latent image are indistinguishable from the remainder of the latent image layer and, under polarized visualization, a maximum contrast between the first latent image and the remainder of the latent image layer is actualized at a different orientation of polarized visualization than a maximum contrast between the second latent image and the remainder of the latent image layer.
According to a further feature of the present invention, there is also provided, a method for producing two latent images comprising the steps of: (a) treating an initially photostable anisotropic polymer having a stretch direction, with a solution containing a photoactivator agent so as to render at least part of the polymer sensitive to radiation; (b) selectively exposing the polymer to a first type of radiation so as to form the first latent image therein; (c) developing the first latent image; (d) selectively exposing the polymer to a second type of radiation so as to form the second latent image therein, wherein the second type of radiation is polarized and the polarization vector of the second type of radiation is oriented at a first angle to the stretch direction; (e) developing the second latent image; and (f) fixing the first latent image and the second latent image, wherein all the above steps are performed such that, when viewed directly, the first latent image and the second latent image are indistinguishable from the remainder of the polymer and, under polarized visualization, a maximum contrast between the first latent image and the remainder of the polymer is actualized at a different orientation of polarized visualization than a maximum contrast between the second latent image and the remainder of the polymer.
According to a further feature of the present invention the step of developing the second latent image and the step of fixing are performed simultaneously.
According to a further feature of the present invention the first type of radiation is ultraviolet radiation and the second type of radiation is infrared radiation.
According to a further feature of the present invention the first type of radiation is infrared radiation and the second type of radiation is ultraviolet radiation.
According to a further feature of the present invention the first type of radiation is polarized and the polarization vector of the first type of radiation is oriented at a second angle to the stretch direction during the step of selectively exposing the polymer to a first type of radiation wherein the first angle differs from the second angle.
According to a further feature of the present invention the first angle is between 30 degrees and 60 degrees.
According to the teachings of the present invention there is also provided the step of laser die cutting the polymer to form a label.